My DS made it to 6 mths on milk, but it was formula from 4 months. He was breast fed from birth, then I swapped to formula and it made a huge difference in the length of time he would go between feeds, and his general contentment.
I'm the only person I know who waited till 6 months though. Out of those babies, there were a couple who I felt were ready -including one who just swiped things off her Mum's plate and ate it - but I don't really know why the others started so early. From what they said, their babies didn't drink that much milk ( not as much as the 40 oz a day my DS had) and slept fine, so why they started early is a mystery to me.
For what it's worth, the babies I know who had trouble sleeping, didn't sleep any better until they were about 7 months and properly established on 3 proper meals a day, with meat, carbs etc. Spoonfuls of rice, veg etc, seemed to make little difference, IME.
I agree with others that milk is more calorific than veg etc, so more milk may be the way to go. I know that's easy for me to say though.
Good Luck with whatever you decide - only you know your baby, and I'm sure you will do what is best for you both.